1. Field of the Invention
This invention generally relates to optical systems for inspecting wafers or reticles with increased resolution. Certain embodiments relate to inspection systems that include a liquid disposed between an optical component of the system and a specimen.
2. Description of the Related Art
As the dimensions of transistors and related structures on semiconductor wafers continue to shrink, the need to detect and identify defects on the wafers and on the reticles used in the lithography process to fabricate the wafers continues to increase. The size of these defects follows the size of the structures on the wafers with a defect about half the minimum dimension on the wafers being considered a “killer” defect that can cause the device to fail. The current minimum dimension on semiconductor devices is about 100 nm with research and development work being conducted on 70 and 50 nm structure sizes. Wafer yield is a function of the number and type of these killer defects, and for more efficient processing and profitability, the number of defects needs to be minimized by first identifying their existence on the wafer and then correcting the root cause. Thus, the detection of defects on both the wafer and reticle is a critical function in current semiconductor fabrication facilities (fabs).
Current fabs utilize various optical inspection tools to locate these defects and classify them into categories, which are related to their root cause. As the size of the defects decreases, the need for higher sensitivity inspection and review tools follows. Generally, sensitivity is related to optical resolution for techniques, which use microscopes to examine the wafers and reticles for defects. Resolution is defined as: Resolution=λ/n(NA), where λ is the wavelength, n is the index of refraction, and NA is the numerical aperture of the optical system at the object. The NA of a typical inspection or review optical tool is currently in the range of 0.90 to 0.95 with a maximum theoretical value of 1.0, so little opportunity exists to increase resolution with this variable. The cost of increasing the NA beyond 0.90 is very expensive due to the difficulty in controlling optical aberrations. The minimum wavelength of current inspection and review tools is in the range of 248 to 266 nm, but the lack of inexpensive continuous sources with wavelengths below this variable is too expensive to consider. The final variable is the index of refraction, which can be a fluid disposed between the surface of the object to be inspected and the last optical surface of the inspection optical system. The higher the index, the better the resolution and smaller defects can be detected, and the increased resolution aids in their classification.
Therefore, it would be advantageous to increase the resolution of optical systems by increasing the index of refraction of the fluid between the object's surface and the last surface of the lens thereby increasing the system's sensitivity without increasing the NA and/or decreasing the wavelength of illumination thereby alleviating the design, manufacture, and test difficulties associated with high-resolution optical systems in addition to reducing the cost of such optical systems.